test-lib: unconditionally enable leak checking

test-lib: unconditionally enable leak checking

Over the last two releases we have plugged a couple hundred of memory
leaks exposed by the Git test suite. With the preceding commits we have
finally fixed the last leak exposed by our test suite, which means that
we are now basically leak free wherever we have branch coverage.

From hereon, the Git test suite should ideally stay free of memory
leaks. Most importantly, any test suite that is being added should
automatically be subject to the leak checker, and if that test does not
pass it is a strong signal that the added code introduced new memory
leaks and should not be accepted without further changes.

Drop the infrastructure around TEST_PASSES_SANITIZE_LEAK to reflect this
new requirement. Like this, all test suites will be subject to the leak
checker by default.

This is being intentionally strict, but we still have an escape hatch:
the SANITIZE_LEAK prerequisite. There is one known case in t5601 where
the leak sanitizer itself is buggy, so adding this prereq in such cases
is acceptable. Another acceptable situation is when a newly added test
uncovers preexisting memory leaks: when fixing that memory leak would be
sufficiently complicated it is fine to annotate and document the leak
accordingly. But in any case, the burden is now on the patch author to
explain why exactly they have to add the SANITIZE_LEAK prerequisite.

The TEST_PASSES_SANITIZE_LEAK annotations will be dropped in the next
patch.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>